Placental tissue is abundantly available as a discarded source of many potentially useful cell types including a type of multipotent cell called placental-derived cells. Although discarded at parturition as part of the placental membranes, lineage analysis shows that, the epithelial layer of the amnion, from which such multipotent cells can be isolated, is uniquely descended from the epiblast in embryonic development. The epiblast contains the cells that will ultimately differentiate into the embryo and cells that will give rise to an extraembryonic tissue, the amnion. Thus far, only four cell types have been described in the literature as being pluripotent. These are the inner cell mass (ICM) of the pre-implantation embryo, which gives rise to the epiblast, the epiblast itself, embryonic stem (ES) and embryonic germ cells (EG). Thus, identification, purification and propagation of a multipotent cell population from discarded amnion tissue would provide an extremely valuable source of stem cells for replacement cell therapy.
With an average yield of over 200 million cells per placenta, large numbers of cells are available from this source. If these cells were to become useful cells for transplantation medicine, they could provide a nearly inexhaustible supply of starting material in every part of the world. No stem cell source provides such a large starting population of cells, and collection does not require an invasive or destructive procedure. Furthermore, there are no ethical, religious or social issues associated with these cells as the tissue is derived from the placenta.
Another important consideration in stem cell therapies is immune tolerance. In humans, the protein expression of the cell surface marker HLA-G was originally thought to be restricted to immune-privileged sites such as placenta, as well as related cells, including some isolated from amniotic fluid, placental macrophages, and cord blood, thus implicating its role in maternal-fetal tolerance (Urosevic, M. and Dummer, R. (2002) ASHI Quarterly; 3rd Quarter 2002:106-109). Additionally, studies involving heart-graft acceptance have suggested that the protein expression of HLA-G may enhance graft tolerance (Lila, N., et al. (2000) Lancet 355:2138; Lila, N. et al. (2002) Circulation 105:1949-1954). HLA-G protein is not expressed on the surface of undifferentiated or differentiated embryonic stem cells (Drukker, M, et al. (2002) PNAS 99(15):9864-9869). Thus, it is desirable that stems cells intended for cell-based therapies express HLA-G protein.
Placental-derived cells have been shown to secrete many cytokines and growth factors including prostaglandin E2, PGES, TGF-β, EGF, IL-4, IL-8, TNF, interferons, activin A, noggin, bFGF, some neuroprotective factors, and many angiogenic factors (Koyano et al., (2002) Develop. Growth Differ. 44:103-112; Blumenstein et al. (2000) Placenta 21:210-217; Tahara et al. (1995) J. Clin. Endocrinol. Metabol. 80:138-146; Paradowska et al. (1997) Placenta 12:441-446; Denison et al. (1998) Hum. Reprod. 13:3560-3565; Keelan et al. (1998) Placenta 19:429-434; Uchida et al. (2000) J. Neurosci. Res. 62:585-590; Sun et al. (2003) J. Clin. Endocrinol. Metabol. 88(11):5564-5571; Marvin et al. (2002) Am. J. Obstet. Gynecol. 187(3):728-734). Many of these cytokines are associated with wound healing and some have been credited with contributing to scarless healing in the fetus. Fetal skin has much more effective repair mechanisms than adult skin and, once wounded, it is able to heal without the formation of scars. This capability does appear to require the fetal immune system, fetal serum, or amniotic fluid (Bleacher J C, et al., J Pediatr Surg 28: 1312-4, 1993); Ihara S, Motobayashi Y., Development 114: 573-82. 1992). Such abilities of fetal tissue have led to the suggested use of compounds produced by fetal tissue for regenerating and/or improving the appearance of skin (see, for example, US 2004/0170615, which is incorporated by reference in its entirety herein).
Approximately 50 million surgical procedures are performed in the United States each year. An additional 50 million wounds result from traumatic injuries. Subsequent acute wound healing failure at any anatomic site results in increased morbidity and mortality. Non-limiting examples of acute wound failure include muscle, fascial and skin dehiscence, incisional hernia formation, gastrointestinal fistulization and vascular anastamotic leaks. Besides the immediate functional disability, acute wounds that fail usually go on to form disabling scars.
Incisional hernias of the abdominal wall provide an excellent paradigm to study the mechanism and outcome of acute wound healing failure. Large, prospective, well-controlled studies have shown that 11-20% of over 4 million abdominal wall fascial closures fail leading to ventral incisional hernia formation. Even after repair of acute wound failure, recurrence rates remain as high as 58%. Improvements in suture material, stitch interval, stitch distance from the margin of the wound, and administration of prophylactic antibiotics to avoid infection significantly decreased the rates of clinically obvious acute wound dehiscence, but only led to small decreases in the rates of ventral hernia formation and recurrence. The introduction of tissue prostheses, typically synthetic meshes, to create a tension-free bridge or patch of the myo-fascial defect reduced first recurrence rates significantly, supporting the concept that mechanical factors predominate in the pathogenesis of recurrent hernia.
Traditional surgical teaching is that laparotomy wound failure is a rare event, with reported “fascial dehiscence” rates clustered around 0.1%. One prospective study found that the true rate of laparotomy wound failure is closer to 11%, and that the majority of these (94%) go on to form incisional hernias during the first three years after abdominal operations. This is more in line with the high incidence of incisional hernia formation. The real laparotomy wound failure rate is therefore 100 times what most surgeons think it is. In simplest terms, most incisional hernias are derived from clinically occult laparotomy wound failures, or occult fascial dehiscences. The overlying skin wound heals, concealing the underlying myofascial defect. This mechanism of early mechanical laparotomy wound failure is more consistent with modern acute wound healing science.